Prior art systems for vehicle route guidance have used vehicle speed, the distance to a required maneuver, the angle of the connected road segment (the difficulty of the turn), assumed models of vehicle performance, and various driver preference factors for use in providing a user with route guidance, such as maneuver instruction timing.
At least one prior art system, Davis et al. U.S. Pat. No. 5,177,685, suggests taking into account the type of road in order to estimate the distance required to decelerate and provide the driver with an instruction to slow down enough in order to perform the required maneuver in a safe unhurried manner. This apparently is accomplished by modeling the vehicle performance and estimating a coefficient of friction of the road by comparing the applied braking force with the resulting deceleration (see col. 21, lines 64-8). However, this only provides a very rough estimate of a road condition which may or may not be an accurate representation of the actual environmental travel conditions that exist.
Prior art systems require the use of physical vehicle characteristics in estimating the amount of time or distance that a vehicle requires to perform a maneuver. This use of vehicle performance characteristics may vary greatly from vehicle to vehicle. Therefore, prior art systems do not allow for easy installation from one vehicle to another and may, in fact, require recalibration because a vehicle's performance changes over time.
To this point the prior art has disregarded important factors related to the environmental travel conditions that may exist for a user when providing route planning and route guidance. Therefore, it would be desirable to provide a system wherein the route planning and guidance that is performed takes into account and is adjusted for current environmental travel conditions such as the time of day, the amount of precipitation falling, the temperature, and other environmental conditions that may affect a user's travel along a given route. By detecting and quantifying factors, such as those listed above, a navigation system can be implemented that is independent of a vehicle's performance characteristics but still compensates for changing environmental travel conditions. In addition, these type of environmental factors could be used to override normal factors because ignoring severe weather conditions could render maneuver instructions useless or even dangerous.